1. Field of the Invention
The present invention relates to polyester resins with high heat resistance and insulated wires to which such resins are applied, and more particularly to polyester resin compositions with high heat resistance and insulated wires using them, which are excellent in tensile elongation properties after heat treatment.
2. Description of Related Art
Conventionally, materials including a polyvinyl chloride (PVC) resin have commonly and widely been used as an electrical insulator. Such insulating materials containing PVC provide advantages of excellent properties in practical use and low cost. However, they have an environmental pollution problem associated with waste disposal because, e.g., they release a gas containing chloride during incineration. Thus, in recent years, there has been a need for an alternative material that can replace PVC.
On the other hand, in the transportation industry such as automobiles and trains, there is a growing requirement for energy conservation by reducing the weight of a vehicle body and by saving space for electrical wiring in a vehicle, which in turn causes a demand for lighter and thinner wires. In order to meet such demand for lighter and thinner wires, however, use of conventional PVC materials presents a problem in that it cannot satisfy the required properties such as flame retardancy and abrasion resistance.
Polyester resins are a type of general-purpose engineering plastic polymers. They, particularly polybutylene terephthalates (PBTs), are crystalline polymers and excellent in heat resistance, mechanical strength, electrical properties, chemical resistance and moldability. Also, they have properties of low water-absorbing as well as excellent dimensional stability, and readily achieve flame retardant properties. These features allow them to be applied to a wide variety of fields such as automobiles, electrical systems, electronics, insulating materials and office automation systems (e.g., JP-B-2968584, JP-B-3590057, JP-A-2002-343141 and JP-B-3650474).
As the polyester resins, there are known: polyester resin compositions including: a polyolefin system resin; a vinyl aromatic compound/conjugated diene compound block copolymer; an olefin copolymer containing an epoxy group; and a vinyl copolymer (e.g., JP-A-2003-213112 and JP-A-2003-221498). It is also known: resin compositions including a polyester resin, a styrene system elastomer and a nitrogen containing compound (refer to JP-A-2003-226798).
Such general-purpose engineering plastic polymers having the above-mentioned features are expected to provide lighter and thinner wires while maintaining flame retardancy and abrasion resistance. While there has been a demand for thinner wire coatings for vehicle use from the viewpoint of energy and ecology conservation, the thickness requirement has been further toughened from approximately 0.5 mm to a lesser thickness (for example, below 0.3 mm). Furthermore, coating materials for vehicle-use wires are required to maintain their tensile properties after heat treatment.
However, polyester resins such as polyethylene terephthalates and polybutylene terephthalates are crystalline polymers, and therefore undergo crystallization by heat treatment, thus significantly degrading mechanical properties, particularly tensile elongation properties, after the heat treatment.